1. Field of the Invention
The present invention relates to methods for fixing an FG magnet used in a rotation frequency detector for the detection of rotation frequency of a drive motor for example in a floppy disk device.
2. Description Of the Prior Art
In known methods for fixing an FG (frequency generator) magnet in this type of rotation frequency detector, the FG magnet and a rotor frame are fixed to each other such that: the gap formed between the FG magnet and the rotor frame is charged for example with adhesives; a hole is bored through the rotor frame; or the rotor frame is sandwiched.
FIG. 4 shows a section of a conventional fixing method for the FG magnet by means of adhesion. Referring to FIG. 4, numeral 2 denotes a rotor frame and numerals 30 and 30' denote forced out adhesives which must be removed.
FIG. 5 shows a section of a fixing method for the FG magnet based on conventional monolithic forming. Since, in this configuration, the gap between a substrate 11 and the lower surface 3' of the FG magnet is reduced as the rotor frame 2 is to be sandwiched, the rotor frame must be raised in height as a whole and the thickness of the motor is increased.
In the above described conventional fixing methods for the FG magnet, there are such problems as that:
(1) in the case of using adhesives, it is necessary to hold and stabilize the FG magnet and the rotor until the adhesives are firmly fixed, and, in addition, adhesives that are forced out from the rotor frame must be removed;
(2) in the case of boring a hole through the rotor frame, leakage flux of the main magnet is increased; and
(3) in the case of forming the FG magnet so as to sandwich the rotor frame, the thickness of the FG magnet in the direction of its height exceeds that of the rotor frame and the motor as a whole cannot be made thinner.